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Simple STP Convergence

I'm trying to figure out how STP(802.1d) would react in the case of a new root bridge being added to a stable topology. The books talk about direct/indirect failues, but not really additions/new links.
-An example: This network is convernged and the blocking port is the X by switch C. (A is the root)


        /     \
       /       \

-Lets say we were going to add a new root switch to switch A:


        /     \
       /       \

-So now switch Z is the root... What would be the downtime here? I am unclear on what ports would have to go through the Learning and Listening stages..if any.

-My Thoughts..
1) Switch A recives a better BPDU so it gives up its root status to switch Z and forwards the better BPDU down to C and B
2) Switch B and C see the different BPDU and replace their old BPDU with the new one.
-Would anything have to go through the Listening and Learning states?
Thank You!  

  • LAN Switching and Routing
Cisco Employee

Simple STP Convergence

Hi Jeremy,

Good point. Let's try to think. SO first port which will go through listening/learning state is the one Between switches A and Z once it will be enabled. Then new BPDus from Z will start to flow.

Switch A will do calculation and select root port to be A-Z one no additional transituion needed. It then will start sending new BPDUs out of ports Ab and AC. Again no transition as those ports were forwarding before.

Switches B and C will receive new superiour BPDUs on root ports -  will do calculations and leave those ports as root - again no transition between sates. Interesting happening when these BPDUs will be sent on ports BC and CB. So these both ports being non-root will receive superior BPDUs. I guess this should immidiately bring those in learning states. AFAIK Better BPDU on a non-root port = listening/learning/forwarding transition. During that they will again decide which side to block.

SO imho there will be two transitions 1) port AZ, 2) Later ports BC and CB.

Open to comments as that is just what I think.